If you are familiar with Log Analytics you will know that you can delete custom field/column directly from the UI as you can see below:
For example I can delete custom field _ResourceId_s for MyLog0001_CL table. That option has been there for quite some time. A couple of years ago an API was made available so you can delete specific data from a table. This was also know as the purge API. The purge API is basically deleting data based on KQL query. With these APIs you could delete specific fields and data but the table that was created by custom log will still be in the Log Analytics.